Hollow fibres are used in a large variety of water filtration devices, be it large municipal water plants or portable water filtration devices.
One possible hollow fibre configuration for water filters is disclosed in U.S. Pat. No. 4,435,289, where porous hollow fibres are supported and sealed by a hardened resin located at both ends of the fibres. Water flows into the inner volume of the fibres at the supported open ends and is filtered when flowing through the membrane wall of the hollow fibres. This flow is an inside-out flow, and filtrate will accumulate in the inner volume of the fibres. Such fibres are cleaning by forward flushing water through the inner volume of the fibres, possibly combined with a backflush as disclosed in International patent application WO 2008/101172 by Vestergaard Frandsen assigned to Vestergaard SA.
Another widespread configuration comprises a bent bundle of fibres with both ends of the bundle being supported in a head, from which the fibres extend downwards into a chamber, from which water is supplied for filtering. This principle is used in large or small water installations, for example as disclosed in U.S. Pat. Nos. 4,636,307, 5,160,673, or 7,081,201.
This principle is also disclosed in connection with personal drinking straws, such as in European patent No. EP2235502B1. This drinking straw has a mouthpiece for sucking water though the straw and a bundle of bent hollow, microporous fibres, which are supported with both ends in a head just below the mouthpiece. By suction action from the mouth of the user, the water flows from the outer side of the straw through the membrane walls of the hollow fibres and into the inner volume of the fibres. From the inner volume of the fibres, the filtered water is released in an upward direction through the open ends in the head and through the mouthpiece above the head.
The drinking straw as disclosed in European patent No. EP2235502B1 suffers from a general problem encountered with such filters as explained further in the following. The hollow fibres are hydrophilic in order to be able to transport water efficiently through the membranes. The hydrophilicity implies that air can not, or only hardly, traverse the membrane walls when these are wet. The consequence is a risk for air trapping in the volume around the fibres, which decreases the water flow, as the trapped air prevents an efficient water flow through the membranes.
This problem is well known and solutions to this problem have been proposed, for example by including a number of hydrophobic fibres, as disclosed in the above mentioned U.S. Pat. No. 4,636,307. However, this solution is production-wise more complicated and expensive.
It would be desirable to provide a simpler solution.
Instead of using bent fibres that extend into an upstream chamber, the fibres could have an open end supported in a head and a closed end extending into an upstream water chamber, for example as disclosed in European patent application EP0938367 and as also mentioned in European patent No. EP2235502B1. The principle is analogous to the one just described and encounters the same problem.
A slightly different configuration is disclosed in US patent application No. 2004/078625, where two bent, hollow fibre bundles are provided in a single housing with the bent parts facing each other. Water flows downwards through the upper, open, potted ends of the hollow fibres and through the bent membrane walls of the fibres. As a redundant measure, the second filter is traversed by an outside-in flow after which the water leaves the filter through the open ends of the second filter. This system suffers from the fact that air, which is transported through the first filter into the intermediate chamber between the two filters, may accumulate and prevent a proper liquid flow.